1. Field
The field of the invention is retorting apparatuses and methods for removal of mercury from ores and mercury contaminated materials.
2. State of the Art
Mercury is a toxic material to many living organisms, including humans, and its presence in ores or in other materials constitutes a hazard during processing of such materials. The mercury contained in such ore is generally in elemental form, or alloy form (amalgam), or as a sulfide, all of such forms which are easily vaporized. Since the refining of such ores often involves the application of heat, the mercury tends to change to highly toxic vapor.
Removal of the mercury by retorting involves the application of heat to the ore or other mercury bearing material to cause vaporization. The ore is heated in a reduced pressure environment to accelerate the vaporization. Such process has typically been a relatively slow, batch type process wherein the mercury bearing material is thinly spread upon trays or pans within a heating oven to minimize the distance the mercury must migrate through the ore. The oven must be shut down and allowed to thoroughly cool before the pans can be removed and replaced with fresh, unretorted material. Other processes include placing the mercury bearing material within a vacuum container which is then heated while the material is stirred by a plurality of rotary paddles. While this process is quicker, it is still a batch process requiring the loading and unloading of batches of material.
U.S. Pat. No. 5,569,154 issued to Navetta discloses a mobile apparatus and a method for continuous processing of mercury laden soil to remove mercury from the soil. The apparatus comprises a screw desorber which includes an elongate closed shell and a motor driven hollow screw which rotates therein, a partial vacuum being maintained therein by means of a vacuum pump. Mercury laden soil enters the screw desorber at one end thereof through a first airlock valve which passes the soil without substantial loss of vacuum therein and travels therethrough under urging of the rotating hollow screw. A burner having a flame directed into the hollow screw incrementally heats the soil to first vaporize mostly water and then mercury contained therein, which vapors and air is withdrawn from the screw desorber at two different points, the first with vapor comprising primarily water vapor and the second downstream therefrom comprising mostly mercury vapor. The soil exits the screw desorber through a second airlock valve at the opposite end thereof which again passes the soil without substantial loss of vacuum therein. The vapor and any air removed from both points pass through respective cyclone cleaners to remove any dust contained therein, and then respective water and mercury condensers, and mercury traps to separate the mercury from the water. Such apparatus requires a closed shell and airlock valves to maintain the vacuum therein. The method comprises the steps of adding soil to one end of the screw desorber under such a vacuum, moving the soil from one end of the screw desorber to the other whereby the temperature of the soil is increased so as to cause mercury to vaporize from the soil, removing the vaporized mercury which is then condensed to form liquid mercury, and removing the soil from the screw desorber. However, such apparatus requires a closed shell which is airtight and does not function in a true continuous process since material does not enter and exit in a continuous manner through the airlock valves. No seal is formed between the screw and the shell.
U.S. Pat. No. 1,599,372 issued to C.J. Reed discloses an apparatus and a method for continuous processing of solid materials by applying heat thereto to effect mechanical separations such as by vaporizing liquids contained therein and chemical reactions. The apparatus comprises one or more elongate, generally horizontally disposed tubes through each of which a motor driven hollow screw extends which rotates therein. The tubes are connected in series one above the other at a slight incline by a plurality of upright tubes, the screws thereof being driven by respective inter-connecting belts or chains, and the interiors of the screws connected by means of respective end shrouds for air flow therebetween. The material enters the topmost tube from a hopper and travels laterally and downwardly through the tubes. A center tube is heated by an external burner assembly and by an internally directed burner flame. A vacuum pump with tank system pulls a vacuum in the centermost tubes, draws offgasses including vaporized mercury therefrom the centermost tubes, and condenses mercury in the tank. Another system circulates air in the tubes adjacent the center most tubes counter to the flow of material therein to exchange heat between entering and exiting material, and includes a condenser and tank to remove water vapor. The centermost tubes which are under vacuum and the adjacent tubes which are not under vacuum are separated from airflow therebetween by the material in the upright tubes interconnecting such tubes. The retorting method comprises the steps of causing mercury bearing ore to move progressively from a lower level to a higher level into and out of a heated zone against a stream of air. While such apparatus functions as a continuous process, it is very complex and bulky, and the efficiency thereof is likely inherently poor due to the extensive lengths of tubes over which to lose heat. No seal is formed between the screw and the shell.
There is a need for an effective mercury retorting apparatus, system, and method of retorting mercury bearing materials using elevated temperatures and lowered pressure which is a continuous rather than a batch type process.